海洋地质与环境重点实验室知识库

大陆裂解是威尔逊旋回中的重要一环，其裂解结构和控制因素是国际研究的前沿。大陆裂解的模式主要分为单剪和纯剪两个端元模型，近年来顺序断层的发育也被用于解释不对称性被动陆缘的形成机制。南海张裂陆缘形成于新生代，具有规模小、年龄小和保存完整的优点。其中，南海西南次海盆的陆缘宽度最大，洋盆规模最小，破裂时间最晚，共轭陆缘结构保存最为完整，是研究大陆裂解结构及控制因素的最佳区域。本次研究利用在南海西南次海盆南北陆缘采集的高分辨率多道地震数据，结合搜集到的海底地震仪（OBS）数据、热流数据和重力数据开展对南海西南次海盆陆缘构造特征等的形态学分析，构造沉降和伸展减薄等的运动学计算，对陆缘的形成机制利用数值模拟方法从动力学角度进行了探讨。得到以下结论：

(1) 地震剖面的层序识别与划分结果显示，北侧陆缘的沉积层厚度总体大于南侧陆缘，而两侧陆缘的裂谷期沉积层厚度均大于漂移期与后漂移期沉积厚度之和。西南次海盆陆缘在研究区的破裂不整合界面的年代被识别为18.5 Ma，地震剖面显示的浅部断层规模相近且呈分布式发育，未能识别明显的拆离断层。断层活动的主要阶段在18.5 Ma之前。北侧陆缘发育有多个地垒地堑构造而南部陆缘发育多个南断北超的半地堑构造。

(2) 考虑区域均衡作用的挠曲悬臂梁模型显示，南海西南次海盆北侧陆缘的拉张因子大于南侧陆缘。综合盆地模拟得到的断层统计显示，北侧陆缘的断层倾角小但偏移量大，而南侧陆缘的断层则表现为倾角大但偏移量小，故南海西南次海盆南北陆缘在裂陷期伸展程度不同且早期张裂的断层样式具有差异性。OBS剖面显示伸展减薄过程中上下地壳发生了解耦。

(3) 与典型裂陷盆地的两段式构造沉降特征不同，西南次海盆共轭陆缘的构造沉降均具有三段式特征：裂谷期的慢速沉降阶段、漂移期的极慢速沉降/隆升阶段和后漂移期的快速沉降阶段。陆缘构造沉降中心随着构造演化从近端陆缘逐渐迁移到了远端陆缘。综合研究认为，下地壳流动补偿了上地壳脆性断裂导致的地壳减薄，从而使得裂谷期构造沉降速率较低。漂移期（海底扩张）时，上地壳的脆性断裂作用减弱，同时在远端陆缘的颈缩区形成次级地幔对流，导致陆缘温度升高浮力变大，且具有抬升作用，使得漂移期构造沉降极其缓慢甚至隆升。后漂移期次级地幔对流效应消失，同时沉积物总厚度变大，驱动下地壳向近端陆缘流动。热沉降、下地壳流及次级地幔对流的消失共同导致了后漂移期的加速沉降。

(4) 动力学数值模拟结果显示下地壳强度越弱，则破裂所需要的时间越长，形成的伸展陆缘宽度越大。综合研究认为南海西南次海盆陆缘下地壳强度小于东部次海盆陆缘，且西南次海盆陆缘下地壳强度具有空间差异性，这种东西差异可能是西南次海盆陆缘破裂存在穿时性的原因。西南次海盆陆缘的伸展模式类似于宽裂谷模式。

The continental breakup is an important part of Wilson cycle，and its rifted structure and the control factors are the leading edge of international study. The stretch patterns are divided into two end models, pure shear and simple shear, and the developments of sequence faults are also applied to interpret the mechanism of asymmetry passive margin formation. The margins of South China Sea (SCS) were formed by Cenozoic extension of the South China continental margin, and it is small, young and well-preserved. The southwest sub-basin (SWB) with the smallest oceanic basin, the widest margins, the latest breakup and the best preserved margins is the best area to study the rifted structure and control factors of continental breakup. This study finishes kinematics analysis about structure, tectonic subsidence, stretch and thinning based on a high resolution seismic profile, which runs across the SWB conjugate margins, OBS data, heat flow data and gravity data. This thesis also discusses the dynamics of the margin formation using the dynamically numerical modelling. The conclusions are listed as follows:

(1) Based on the interpretation of seismic profile, this study finds the greater sediment thickness in the northern continental margin than the southern margin, greater sediment thickness in the synrift period than the gross in spreading and post-spreading stages. The faults of similar scales nearly well distribute along the margins, with no obvious detachment fault, and the main faulting period is before 18.5 Ma. Many horst and graben structures develop in the northern continental margin, while half-grabens faulted in the south and overlapped in the north dominate in the southern continental margin.

(2) From flexural-cantilever model, which considers the regional isostatic effect, the stretch factors in northern margin is larger than that in southern margin. The statistics of faults, which are from kinematic simulation, show that faults in northern margin have smaller dips and larger offsets than these in southern margin. Therefore, the stretch degrees of conjugate margins are different during rift stage. The OBS profiles show the decoupling of the upper and lower crust during the extensional thinning process.

(3) The tectonic subsidence on conjugate margins are divided into three stages: slow subsidence during synrift stage; extremely slow subsidence/uplift during spreading stage; accelerated subsidence during post-spreading stage, which is different with two-stage tectonic subsidence in classical rifted basins. The subsidence center shifts from the proximal margin to distal margin with the tectonic evolution. The lower crust of the SWB margins was relatively weak, which would flow to the center of faulting and compensated for the upper crustal thinning caused by brittle faulting during the synrift period and thus reduced the tectonic subsidence rate. During the spreading stage, faulting attenuated rapidly and small-scale secondary mantle convection occurred at the necking zone, which raised the continental margin isotherms and increased the buoyancy. Simultaneously, secondary mantle convection lifted the overriding crust, so the overall subsidence rate decreased sharply or even reversed to uplift. After seafloor spreading, the effect of mantle convection vanished and vast sediment loading drove the lower crust to flow landward. Thermal relaxation, lower crust flow and the vanish of secondary mantle convection together lead to a rapid subsidence during this stage.

(4) From the results of numerical simulation about dynamics, this study finds that if the lower crust is weaker, the time needed for rifting before breakup is longer and the rifted margin is therefore wider. Based on previous studies of other scientists, this study proposes that the lower crust in SWB margins is weaker than that in east sub-basin margins, and the lower crustal strength in SWB margins has a change from east to west, which may be the main reason for diachroneity of breakup in SWB margins. The stretch pattern of SWB margins is more like the wide rift mode.

目 录

第1章 绪论... 1

1.1 选题背景及意义... 1

1.1.1 被动陆缘伸展模式研究的意义... 1

1.1.2 南海陆缘构造的空间差异性... 1

1.1.3 南海西南次海盆及陆缘特征与研究的必要性... 2

1.2 国内外本学科领域的发展现状与趋势... 2

1.2.1 被动陆缘分类... 2

1.2.2 被动陆缘的构造单元划分... 5

1.2.3 伸展模式研究... 6

1.2.4 南海伸展模式研究... 8

1.3 研究内容... 9

1.4 研究方法... 10

1.5 技术路线与实验方案... 11

第2章 地质背景... 13

2.1 大地构造背景... 13

2.2 南海西南次海盆及陆缘地质背景... 14

2.3 南海西南次海盆及陆缘地球物理特征... 14

2.3.1 重力异常... 14

2.3.2 磁力异常... 15

2.3.3 热流... 17

第3章 南海西南次海盆陆缘沉积及构造分析... 19

3.1 所用数据... 20

3.1.1 二维多道地震数据... 20

3.1.2 OBS数据... 20

3.1.3 重力数据... 20

3.2 分析方法... 21

3.2.1 地震解释... 21

3.2.2 重震联合反演... 22

3.3 结果... 22

3.3.1 浅部沉积及构造特征... 22

3.3.2 深部结构分析... 23

3.3.3 重力反演结果... 25

3.4 本章小结... 26

第4章 南海西南次海盆陆缘伸展减薄特征... 27

4.1 挠曲回剥方法与挠曲悬臂模型介绍... 27

4.1.1 挠曲回剥方法介绍... 27

4.1.2 挠曲悬臂梁模型介绍... 29

4.2 伸展减薄分析数据... 30

4.2.1 地震数据及层序深度... 30

4.2.2 层序格架及年代数据... 31

4.2.3 岩性数据... 32

4.2.4 古水深与相对海平面变化... 33

4.3 结果与讨论... 36

4.3.1 盆地模拟结果... 36

4.3.2 OBS数据分析结果... 41

4.3.3 讨论... 42

4.3.4 误差分析... 44

4.4 本章小结... 44

第5章 构造沉降分析及机制探讨... 47

5.1 构造沉降计算方法... 47

5.2 参数选取与不确定性分析... 50

5.2.1 参数选取... 50

5.2.2 不确定性分析... 50

5.3 构造沉降结果与讨论... 50

5.3.1 北侧陆缘构造沉降特征... 50

5.3.2 南侧陆缘构造沉降特征... 52

5.3.3 构造沉降特征及其机制讨论... 54

5.4 本章小结... 61

第6章 南海西南次海盆陆缘动力学数值模拟... 63

6.1 数值模拟方法介绍... 65

6.2 参数选取... 66

6.3 模拟结果... 67

6.4 讨论... 69

6.4.1 东西次海盆陆缘下地壳流变强度差异... 69

6.4.2 西南次海盆陆缘下地壳强度的空间差异... 70

6.5 本章小结... 73

第7章 结论与展望... 75

7.1 主要结论... 75

7.2 存在问题及下一步工作展望... 76

参考文献... 77

附 录... 87

致 谢... 89

作者简历及攻读学位期间发表的学术论文与研究成果... 91